There are a variety of situations in which it is desirable to repair defective or damaged areas of tubes contained within a surrounding structure, such as a tube sheet. By way of example only, large heat exchangers, particularly the type used as steam generators in power plants, typically employ a tube sheet which is a metal plate that can be of varying thickness and has bores of a suitable diameter in which the tubes are inserted. The tubes are often made of stainless steel or carbon steel and act as conduits for fluid. With the passage of time, the interior surfaces of the tubes tend to become eroded, corroded or pitted and may develop cracks, crevices or other defects. These defects especially tend to arise in the area where the tubes and tube sheet define joints. If these defects are left unattended, they decrease the predictable life expectancy of the heat exchanger and associated equipment and may cause undesirable leaking of fluid.
Known techniques for dealing with these defects involve the insertion of a protective sleeve within the tube in the vicinity of the damaged or defective areas of the tube accompanied by radial expansion of the sleeve through a roller expanding process. This process employs a mechanical implement which is inserted in the sleeve and pressed against the inner surface of the sleeve so as to force the wall of the sleeve to expand radially outward. The force applied to the wall of the sleeve is also typically sufficient to radially expand the wall of the tube. Upon completion of the process, the tube radially contracts somewhat so as to achieve a press fit with the sleeve.
Roller expanding processes, however, have a number of disadvantages. For one, mechanical rolling of the interior surface of the sleeve tends to result in a sleeve having a wall which is undesirably thin in at least certain areas and, therefore, has less of an anticipated useful life. The reason is that the roller expanding process decreases the thickness of the wall of the sleeve not only due to the change in mathematical area caused by the radial expansion, but also due to deformation of portions of the wall. Moreover, roller expanding processes tend to be time consuming. That is, the rollers can only contact a certain area of the sleeve at any given time. Therefore, the rolling must be performed in stages along the length of the sleeve.
The use of rollers also imposes a minimum dimension on the inside diameter of the sleeve in relation to the wall thickness of the sleeve, since it must be possible to insert rollers of suitable strength and rigidity. Roller expanding processes further tend to leave gaps between the outer surface of the sleeve and the tube. Typically, these gaps are caused by the inherent diametric non-uniformities of the sleeve and tube across their respective lengths or by non-uniformities introduced by defects in the tube. In the case of the latter non-uniformity, the roller expanding process tends to simply "bridge over" the defect, rather than fill in the areas with the expanded wall of the sleeve. Additionally, corrosive agents tend to collect in gaps and may eventually corrode the sleeve or tube.
It should, therefore, be appreciated that there has existed a definite need for a method and apparatus for radially expanding and anchoring a protective sleeve within a tube contained within a surrounding structure that sufficiently repairs a defective or damaged area of the tube and better extends the useful life of the tube and surrounding structure.